Mapping salinity

Setting:

The Danish Hydrology Research Center, HOBE wanted to balance the water budget calculations of a catchment area. They suspected the aquifer was discharging into a lagoon via a buried channel and investigated the area to determine the channel’s significance in submarine groundwater discharge (SGD).

Technology:

The SkyTEM 304 was flown at an average speed of 50 kph. The transmitter was set for low (LM)and high (HM) transmitter moments of 5000 and 179000 Am2 respectively. This permitted measurement of 20 unbiased gates (11.7 ?s – 450 ?s) in LM and 21 gates (94 ?s- 8 ms) in HM and maximized the response from the upper layer concurrently while mapping to a depth of over 300 m.

Findings:

The hydrogeological setting was found to be complex as the geology was dominated by stratified deposits with a homogeneous structure of alternating layers. In addition to mapping the stratified geology SkyTEM mapped the buried valleys beneath the lagoon. The system was able to penetrate through the layers to a depth greater than 300m and showed that the valleys were saturated with sea water (see slice at 100-110 m depth). Mapping salinity distribution helped to outline an intricate flow system beneath both the coastal area and the lagoon and answered questions on how and where freshwater was moving. This changed previous assumptions on the heterogeneity of the area.

 

Conclusion:

Because of the extremely high SNR, sensitivity and accuracy are enhanced and subtle differences in resistivity can be resolved. An interesting discovery is that accurate bathymetric and salinity level maps can be produced from the SkyTEM method. It suggests as well that the method can be used for mapping layered features within a water column, such as haloclines.